Plasma discharges can be used to dissociate gases to produce activated gases containing ions, free radicals, atoms and molecules. Activated gases are used for numerous industrial and scientific applications including processing materials such as semiconductor wafers, powders, an other gases. The parameters of the plasma and the conditions of the exposure of the plasma to the material being processed vary widely depending on the application.
For example, some applications require the use of ions with low kinetic energy (e.g., a few electron volts) because the material being processed is sensitive to damage, or because there is a requirement for selective etching one material relative to another. Other applications, such as anisotropic etching or planarized dielectric deposition, require the use of ions with high kinetic energy.
Some applications require direct exposure of the material being processed to a high density plasma. Such applications include ion-activated chemical reactions and etching of and depositing of material into high aspect-ratio structures. Other applications require shielding the material being processed from the plasma because the material is sensitive to damage caused by ions or because the process has high selectivity requirements.
Plasmas can be generated in various ways including direct current (DC) discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by capacitively or inductively coupling energy from a power supply into a plasma. For example, parallel plates can be used to capacitively couple energy into a plasma and induction coils can be used to induce current in the plasma. Microwave discharges can be produced by coupling a microwave energy source to a discharge chamber containing a gas.
Plasma discharges may be generated in a manner such that both the charged species constituting the plasma and the neutral species, which may be activated by the plasma, are in intimate contact with the material being processed. Alternatively, the plasma discharge may be generated remotely from the material being processed, so that relatively few of the charged species comes into contact with the material being processed, while the neutral species may still contact it. Such a plasma discharge is commonly termed a remote or downstream plasma discharge. Depending on its construction, position relative to the material being processed, and operating conditions (gas species, pressure, flow rate, and power into the plasma), a plasma source can have characteristics of either or both of these two types.
Existing remote plasma sources generally utilize RF or microwave power to generate the plasma. Although present sources support many applications successfully, several limitations remain with respect to practical use of these plasma devices. One such limitation resides within the plasma applicator (i.e., the plasma vessel). For example, over time, the plasma applicator can become worn from use. Specifically, reactive species may deposit or etch the walls of the plasma applicator. Current designs do not allow for efficient refurbishment of the applicator due to complex mounting and cooling arrangements. In addition, current plasma applicators are made from processed materials (e.g., channels forming the flow path within the applicator are drilled or etched into a block of starting material). As a result, the channels typically contain a high number of surface defects, which can lead to higher particle/contaminant generation during plasma generation.
Moreover, some applications require the use of highly corrosive gasses and/or plasmas (e.g., F containing gasses and plasmas). These applications require the use of plasma applicators made from expensive materials to process, such as, for example sapphire, that can withstand exposure to the corrosive environment without becoming structurally compromised. Other applications, which do not involve corrosive materials, can be accomplished using an applicator made from a less expensive material, such as, for example, quartz. Current designs are limited to the use of either a corrosive resistant, expensive applicator or a less expensive, corrosive non-resistant applicator. As a result, a user needs to have at least one dedicated plasma source for corrosive applications.
A second difficulty with existing remote plasma sources is removal of the heat generated in the plasma and transferred to the walls of the plasma vessel. This is especially the case when the plasma vessel has a complex shape and when it is composed of a dielectric material for which direct cooling with large quantities of fluid in contact with the dielectric vessel is either undesirable or impractical. In addition, the cooling components used to cool the complex shapes of plasma applicators further limits refurbishment of worn applicators or interchangability of an applicator with a different material type applicator.